This invention relates to improved cadmium negative electrodes for use in galvanic cells, and a process for producing the same. More particularly the invention pertains to flexible cadmium electrodes which are capable of being wound in a jelly-roll configuration for use in rechargeable nickel-cadmium alkaline cells.
The typical known cadmium electrode consists of a mixture of cadmium oxide, cadmium hydroxide, cadmium metal, a minor amount of nickel hydroxide to retard cadmium agglomeration (and capacity fade), and a binder to provide integrity to the mixture and secure attachment to an electrically conductive substrate. In preparing pasted cadmium electrode plates, it has been common practice to use a water soluble or water dispersible resin as the binder for the electrochemically active material. The use of water to blend the electrochemically active ingredients with the binder has several drawbacks however. The water reacts with cadmium oxide to form the corresponding hydroxide which is of lower density. The result is a reduction in capacity of the electrode as well as pollution of process water. The hydroxide form presents the further drawback that it reacts with air on stand to form the undesirable carbonate species. Water in the presence of air also promotes conversion of the cadmium metal to cadmium hydroxide.
Perhaps the best prior art non-sintered cadmium electrodes have been made using polytetrafluoroethylene (PTFE) dispersible resin in water. The PTFE binder holds the electrode mass together by a series of fibrillate fibers throughout the active mass and serves to enhance recombination of oxygen at the negative cadmium electrode on charge and overcharge. It is also known to catalyze this oxygen recombination reaction by impregnating the active mass of the cadmium electrode with sub-microscopic silver particles, as taught in U.S. Pat. No. 3,877,985 (Rampel). Typically this type of Teflon bonded electrode has been prepared according to the teachings of U.S. Pat. No. 3,954,501 (Rampel), the disclosure of which is herein incorporated by reference. In this process cadmium oxide is hydrated to typically about 70 percent. Nickel hydroxide up to about 2.0 weight percent is also added to retard cadmium agglomeration (densification) as taught in U.S. Pat. No. 3,870,562 (Catherino). Nevertheless, cycle life of sealed nickel-cadmium cells incorporating these cadmium electrodes is limited primarily by the agglomeration phenomenon to about 250 cycles in certain applications. The agglomeration causes a gradual reduction of the active electrode surface area due to densification of the cadmium metal (charge state). Thus, the effective current density gradually increases and the capacity of the cell measured in ampere hours deliverable to a fixed cut-off voltage decreases. For applications requiring long cycle life the usefulness of this known cell is accordingly limited.
Japanese patent publication no. 57-174864 assigned to Matsushita recognizes the disadvantages of transformation of cadmium oxide to cadmium hydroxide when employing water soluble pastes. This patent publication discloses the use of poly (vinyl) alcohol as a binder dispersed in ethylene glycol or propylene glycol non-aqueous solvents. The strength and conductivity of this electrode are improved by adding resin or metal fibers to the paste, however these fibers are acknowledged to increase the difficulty of extruding or slurry coating the electrode substrate as it is pulled through a slit. The fibers used include acrylonitrile-vinyl chloride copolymer, polyamide and polyvinylchloride as resin fibers, and steel, nickel, and nickel plated resin fiber as metal fibers. The capacity of the resultant electrode is disclosed as 75 mAh/cm.sup.2.
Japanese patent publication no. 52-31348 assigned to Furukawa discloses a method for preparing a cadmium electrode including the steps of filling and then drying a paste formed by mixing a powdered cadmium active material, a binder solution and a lower melting point nylon powder composed of a copolymer of nylon 6, nylon 66 and nylon 12, in a substrate, and heating the paste at a temperature exceeding the melting point of the lower melting point nylon whereupon the nylon powder is melted to bind the active material powders. It is believed this method will fail to produce an electrode with satisfactory dispersion of the nylon, and that the in situ melted nylon will mask the activity of the cadmium. Most importantly this reference does not disclose use of a material which will retard cadmium agglomeration during electrode cycling.
The state of the prior art of cadmium electrodes is also represented by the teaching of Japanese patent publication no. 54-106830 (assigned to Furukawa and reported in CA 92:8884m). According to that reference a paste consisting of cadmium oxide or cadmium hydroxide and a binder is applied on to a porous sheet coated with thermally fusible nylon grains, and the pasted sheet dried and conversion treated to prepare cadmium anodes. Nylon grains of 0.2-0.4 mm are disclosed as being blown on a nickel-plated iron sheet and a paste consisting of cadmium hydroxide, nickel powder, short fibers, poly (vinyl) alcohol, carboxymethyl cellulose, and water is applied to the nylon-coated sheet with a reported reduction in the percentage of defective electrodes produced. It is believed the nylon serves as a binder to promote cohesion between the active material and substrate.
It is a primary object of the present invention to provide a cadmium electrode whose life is extended by retarding cadmium agglomeration.
It is another object of this invention to provide a means for imparting anti-agglomerating characteristics to known cadmium electrodes to extend their life, and a simple process to achieve the same including post treatment of prefabricated, known electrodes.
It is another object of this invention to prevent or severely retard hydration of the active material of a cadmium electrode during manufacture and storage.
A further object of the invention is to provide a cadmium electrode employing a novel anti-agglomerating material also serving as a binder characterized by the following properties: imparts improved adhesion and cohesion of the active cadmium material to itself as well as to the electrode substrate; remains stable in the cell environment; the binder does not significantly mask the active material which would otherwise limit high rate performance and gas recombination efficiency; provides flexibility so that the resultant electrode can be wound if desired; and produces an electrode paste which can be readily applied to a substrate by extrusion, slurry coating or other methods adapted to continuous manufacture with a high degree of uniformity of application.
It is still a further object to provide a cadmium electrode for use in a rechargeable cell employing an alkaline electrolyte in which the binder/anti-agglomerant is preferably in the form of a microporous interconnecting polymeric network, and in which an additive is intimately mixed with the active material which functions as an electrolyte wick to maintain electrolyte at the active porous surfaces of the electrode retained in the polymeric network, during charge and discharge of the electrode and which also functions as an anti-compressive agent to maintain the proper porosity and physical dimensions of the electrode during such charging and discharging.
It is still a further object to provide the foregoing electrodes, a simple and effective process for producing such electrodes, and improved rechargeable alkaline cells, namely nickel-cadmium and the like, employing such electrodes.
These and other objects of the invention are met and disadvantages of the prior art solved by employing the electrode plate and its method of preparation according to the present invention as described herein.